High density optical storage, direct access storage devices, are usually circular in shape, with concentric circular, or alternatively spiral, data "tracks" along which data is recorded. In typical optical storage devices, grooves provide the physical identification of, and separation between, adjacent optical device tracks.
The data tracks of typical storage devices consist of "sectors" each having an address part, or "headers", and an information part. The headers are addressable units which serve to identify the relative locations of information within a particular track.
In optical storage devices, tracks may be distinguished by "grooves" and headers may be identified by discrete "pits", each preformed during the manufacturing process on the surface of the optical device substrate. Such an optical storage device offers a "preformatted" structure. That is, a structure having a plurality of grooves and pits that provide tracking and addressing information respectively.
Alternatively, the preformed device pits may comprise both header and digital data contents (actual information recorded within each sector) on the surface of the optical device substrate as is typical for conventional permanent read only or pre-recorded disks. Thus defining a "read only" optical storage device as consisting of the preformed structure offering a plurality of grooves and pits that provide tracking and addressing information as well as digital data contents itself.
The prior art consists of dual depth optical storage devices as optical devices in which the optical device substrate contains both grooves and pits, each disposed on planes parallel to, and in the same direction from, the substrate surface. In one embodiment, the device grooves and pits are integrally formed within the device substrate and downwardly disposed from the plane of the substrate. In an alternate embodiment, the device grooves and pits are integrally formed upon the device substrate and upwardly disposed from the plane of the substrate. In the alternate embodiment the track portions of the optical storage device are separated by reverse grooves, or "ridges", the peaks of which are disposed substantially above radially adjacent data track sections of the optical storage device; and the header portions (and digital information within the sectors in a read only device) comprise reverse pits, or "hills". The device grooves are shallower than the device grooves in the first prior art dual depth structure, and the device ridges are shorter than the device hills in the alternate prior art dual depth structure.
During normal operation, optical storage device grooves facilitate detecting and correcting radial positioning of the read/write head within the storage subsystem. Typically, the radial position is maintained or corrected by means of the "push-pull" or differential method as disclosed in, for example, U.S. Pat. No. 4,363,116. In that method, the optical device is exposed to a radiation "read" beam that is reflected into a zero-order, first-order and higher-order subbeams. The angular difference between the zero-order subbeam and the first-order subbeam being defined as the "phase depth".
The push-pull method utilizes two detectors which receive different orders of the reflected read beam. The difference between the output signals of the two detectors provides information as to the radial orientation of the read beam with respect to a device track (as indicated by a groove or ridge). If the output signals differ, the read beam does not coincide with the central axis of the track, and the difference fed to a servo system acts to correct the read beam orientation.
The prior art illustrates that the push-pull, or difference signal, varies as a function of the relative depth (height) of grooves and pits (ridges and hills) in a dual depth device with the relative depth (height) defining a phase depth. Further, the push-pull signal varies as a function of the relative degree of "flatness" of the "bottom" of pits and grooves (or the "tops" of hills and ridges in the alternate embodiment). Ideally, relatively "flat" pit and groove bottoms (or hill and ridge tops), along with a structure of proper phase depth, result in a sufficiently accurate push-pull signal with constant envelope. That is, a signal having zero amplitude at the central axes of each groove (or ridge) in the optical storage device and maximum amplitude when approximately halfway atop the groove or ridge.
In the manufacture of a dual depth device, a substrate is first coated with a light sensitive photoresist. A light source is selectively attenuated, or otherwise varied in intensity, to provide a desired exposure pattern on the photoresist. The photoresist is then developed to form the desired pattern of grooves and pits. The intensity of exposure determines the resulting relative depth of the device grooves and pits. The resulting device may be used as the optical device itself or serve as a stamper master for replication of the optical device.
The above-described method for dual depth manufacture ideally would result in grooves and pits (ridges and hills) of well controlled relative depths (heights) and flat bottoms (tops). However, the light source attenuation may be imprecise; the light source used to expose the photoresist is Gaussian in distribution and unavoidably decreases in intensity radially from the light source center; and the subsequent development process is narrow. Thus, the resulting groove and pit (ridge and hill) geometry and contour, of the dual depth optical structure, may lack desirable relative depths (heights) and actually have rounded bottoms (tops). The resulting geometry and contour may also adversely affect the push pull signal and lead to erroneous tracking information.
An improvement to the above process is illustrated in U.S. Pat. No. 5,060,223 ('223) to Segawa. '223 defines pits as a function of the wavelength of an incident read beam and refractive index of the device substrate, as well as defining the pitch of device grooves. Another improvement to the above process is illustrated in U.S. Pat. No. 4,469,424 ('424) to Matsui, et al. wherein a monitoring beam is utilized during the development process to more accurately control the depths and widths of grooves and pits to within a predetermined allowable range. Yet another improvement to the above process is illustrated in U.S. Pat. No. 4,732,844 ('844) to Ota et al. wherein two layers of photoresist, separated by an intermediate layer, are exposed and independently removed. The resulting dual depth device offers grooves and pits having more accurately controlled depths and widths. U.S. Pat. No. 4,893,298 ('298) to Pasman, et al. provides still another example of an improvement to the above embodiments wherein track width is defined as a function of the track period. Further, '298 discloses an, embodiment of tracking grooves and data pits, or an alternative embodiment of "ridges" and "hills". Notwithstanding the attempts in the prior art to control the relative depths (heights) and contour of the dual depth optical storage device structure, the resultant device is bound to the practical limitations of the dual depth structure itself. Namely, on the one hand there is the need to reduce the physical size of optical storage devices, while on the other hand compactness is limited to the dual depth optical storage device embodiment.
A practical consequence of dual depth optical device structures having non-ideal optical storage device geometries and contours, results in signal envelopes with varying amplitudes. In a severe case, the amplitude of a tracking error signal may be reduced to below minimum recognizable levels, so that it is impossible to normally perform tracking control.
The prior art illustrates that there are several shortcomings of the dual depth structure described above. Namely, the use of underexposed photoresist results in narrow process and development windows resulting in undesirable groove and pit geometries. The conventional process of manufacture is one wherein the development step is the same for device grooves and pits. As such, a means for control of the pit and groove features is largely dependent on precise control of the exposure step. In order to ensure desired tracking and accessing performance with the conventional manufacturing process, the yield tends to be low. Further, the process is not applicable to glass substrates. Glass substrates offer superior optical and mechanical properties for high performance storage applications.
The processes of dual depth device manufacture and improvements are not suited to optical storage devices comprising both pits and "reverse" grooves, or ridges, because of the limited ability to control two independent features in single developing and etching steps. Prior art methods of manufacture that offer dual depth device embodiments exclusively with either pits and grooves, or ridges and hills. We define bipolar geometries as devices having both pits and ridges, each disposed on planes parallel to, and in opposite directions from, the substrate surface.
EP 0220578 discloses and claims a bipolar patterning process. The disclosed method is based on the use of a process of manufacture for self aligned semiconductor structures having bipolar patterns. The process of manufacture of self aligned bipolar semiconductor devices has been ignored in optical storage device manufacture. In semiconductor devices, a bipolar pattern is used to take advantage of diverse electrical properties associated with heterogeneous layers of semiconductor substrates. No such requirement pertains to optical storage devices. Further, while the process in EP 0220578 works for large lithographic geometries, it is impractical for small geometries required for high performance optical storage devices. That is, while the photoresist as described in EP 0220578 is capable of sub-micron resolution, the optical mask system described in EP 0220578 is not. In addition, reasonable improvements to that optical system (such as improving lens performance by increasing numerical aperture) still will not enable the system to print geometries required for optical storage devices.
As previously noted, attempts have been made to improve known manufacturing processes to yield more desirable groove and pit geometries but cost effective solutions have to date largely eluded researchers.